Ion Adsorption on Modified Electrodes as Determined by Direct Force

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Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control Volodymyr Kuznetsov† and Georg Papastavrou* Department of Physical Chemistry II, Universitätsstrasse, 95440 Bayreuth, Germany

ABSTRACT: Ion adsorption is a charging process that is of central importance for many hydrophobic surfaces, such as gas bubbles, oil drops, and cell membranes. This process has been studied by various techniques and different adsorption mechanisms have been proposed so far. However, different analytical methods seem to indicate the adsorptions of different types of ions, particularly hydroxyl or hydronium ions, at hydrophobic surfaces. In this work, we studied ion adsorption on modified electrodes by direct force measurements with the colloidal probe technique. The electrodes were modified with a self-assembled monolayer (SAM), and their potential was controlled externally by means of a potentiostat. Ion adsorption onto OH- or CH3terminated SAMs was determined as a function of pH and background electrolyte concentration. Charge at the interface was found to result primarily from the adsorption of hydroxyl and hydronium ions, whereas the influence of the background electrolyte (KCl) can be neglected. For the hydronium and hydroxyl ions, we determined the adsorption constants by means of a simple semiquantitative model and found that the adsorption constant of hydroxyl ions is orders of magnitude larger than that of hydronium ions. Furthermore, ion adsorption was found to be much more pronounced for hydrophobic surfaces than for hydrophilic ones.

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adsorption of hydroxyl ions.7,18−26 Ion adsorption at hydrophobic interfaces has been studied theoretically by various groups as well, but without providing a decisive picture so far.12,13,27,28 In this work, we followed an alternative experimental approach to study ion adsorption at hydrophobic and hydrophilic interfaces. Our approach is based on direct force measurements on electrodes modified with self-assembled monolayers (SAMs; cf. Figure 1a). This approach provides several advantages in comparison to standard electrokinetic techniques: First, the diffuse-layer potentials as determined by direct force measurements are unambiguous in relation to the position of the plane of shear. Second, the potential of zero charge (pzc), which is the externally applied potential at which the diffuse-layer potential of the electrode vanishes, depends critically on the presence of adsorbed ions at the interface (cf. Figure 1c). However, in contrast to classical electrochemical methods, by direct force measurements, one can probe the full diffuse layer and not primarily the adsorbed ion layer. Finally, a

INTRODUCTION The interfaces of water with gases, solids, and other immiscible liquids are ubiquitous on Earth.1−4 They are of central importance not only for life and environmental issues but also for many industrial processes.1,2,5 Recently, a number of theoretical and experimental studies addressed the charging mechanisms at these interfaces on a molecular level.6−12 Of particular interest were interfaces between water and hydrophobic phases, such as water/oil, water/lipid, and water/air interfaces. Different surface analytical techniques provided apparently contradictory data regarding the extents to which hydroxyl and hydronium ions adsorb preferentially at hydrophobic interfaces.6−12 Different spectroscopic techniques, which are sensitive to the interface at the molecular level, allow the ions that preferentially adsorb at a hydrophobic interface to be determined.13−17 However, different studies have reported preferential adsorption of either hydroxide ions or hydronium ions, with a majority reporting the latter. By contrast, studies based on electrokinetic techniques, which probe primarily the diffuse layer originating from the interface, show an overall negative sign for the diffuse-layer potential under basic conditions and thus suggest the preferential © 2014 American Chemical Society

Received: January 14, 2014 Published: January 28, 2014 2673

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Figure 1. (a) Schematic representation of the experimental setup to determine the interacting forces between a colloidal probe prepared from a silica particle and a gold electrode modified with a self-assembled monolayer (SAM) terminating in nonionizable functional groups. The electrode is connected as the working electrode (WE) to a potentiostat in a three-electrode electrochemical cell with a counter electrode (CE) and reference electrode (RE). (b) Schematic representation of the three-capacitor model for electrodes modified with a self-assembled monolayer (SAM) including an adsorbed layer of ions and the diffuse layer. This model was used to provide a semiquantitative description of the electrodes’ diffuse-layer properties as determined by direct force measurements. (c) Schematic representation showing how the adsorption of ions onto the SAM at different pH values leads to different potentials of zero charge (pzc) to compensate for the additional charge due to the ions.

solutions 1 M HCl, 1 M KOH, and 1 M KCl (all analytical grade). The pH values of solutions in the basic regime were adjusted directly before the measurements by adding the precalculated amount of KOH to the previously degassed solutions already containing the appropriate amount of KCl to obtain a total ionic strength of 1.2 mM. The final pH values of the solutions were controlled with a pH meter. Preparation of Flat Gold Electrodes. The gold electrodes were prepared as reported recently38 using a procedure based on a modified “template stripping” technique.42 Onto RCAcleaned Si wafers (CrysTec, Berlin, Germany) was deposited a 60-nm-thick layer of gold (99.99% purity) by thermal evaporation (mini-coater, tectra GmbH, Frankfurt, Germany). Directly after evaporation, the 11 × 11-mm-sized glass slides were bound to the gold layer with chemically resistant glue (EPO-TEK 377, Epoxy Technology Inc., Billerica, MA) that was subsequently cured by thermal treatment (1 h, 150 °C). Flat gold electrodes were finally obtained by mechanically separating the glass slides with the gold layer from the wafer. These ultraflat gold electrodes were immediately rinsed with ethanol and then transferred into thiol solutions. Surface modification with thiols was conducted for at least 12h in 1 mM ethanolic solutions of 16-mercaptohexadecane-1-ol or 1hexadecanthiol, respectively. The electrodes with OH-terminated SAMs were rinsed with copious amount of ethanol and then water. The electrodes with CH3-terminated SAMs were rinsed with copious amounts of ethanol and then sonicated twice in fresh ethanol in an ultrasonic bath. The electrodes were immediately mounted in the electrochemical cell and covered with degassed electrolyte solution. Directly afterward, the atomic force microscopy (AFM) fluid cell was closed. The formation of the SAMs was verified by contact angle measurements (OCA 15, Data Physics, Filderstadt, Germany). The contact angles determined (107.8° ± 0.5° for the CH3-

simple model approximating the ion layer as an additional capacitance (cf. Figure 1b), in series with the SAM and the diffuse layer, allows for a quantitative description of the diffuselayer potential as a function of the externally applied potential.29 Various studies of direct force measurements on electrodes under potentiostatic control have been reported so far.30−38 However, in this work, the electrode was additionally modified with a self-assembled monolayer (SAM) that allows the surface of a gold electrode to be rendered either hydrophobic or hydrophilic in a highly defined way.39 In this study, we included only SAMs bearing uncharged functional groups, namely, hydroxyl and methyl groups, that do not ionize as a function of pH. Additionally, the adsorption of hydroxyl and hydronium ions to such SAMs has been studied previously by theoretical methods.27 In previous studies with such electrodes, we concentrated on the diffuse-layer properties as a function of thiol length and ω-functionalization35 or on the resulting adhesion behavior.38 By contrast, in this study, we determined the diffuse-layer properties of SAM-modified electrodes as a function of external potential and pH. So far, comparable SAMs have been examined primarily by streaming potential measurements25,40 or direct force measurements under open-circuit conditions with respect to the influence of ionic strength and pH.41

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MATERIALS AND METHODS Materials. 16-Mercaptohexadecanol-1 (99%, Frontier Scientific) and 1-hexadecanthiol (99%, Sigma Aldrich) were used for electrode modification as received. Solutions of these thiols were prepared with ethanol (p.a.). All aqueous solutions were prepared with deionized water of Milli-Q grade with a resistivity greater than 18 MΩ/cm. The pH values and total ionic strengths of the aqueous solutions were adjusted with stock 2674

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terminated SAM and 22.0° ± 3.0° for the OH-terminated SAM) are in line with the values reported in the literature.43 Preparation of Colloidal Probes and AFM Cantilevers for Force Measurements. Tipless AFM cantilevers (NSC12, Mikromasch, Wetzlar, Germany) were cleaned in a series of solvents (ethanol, acetone, chloroform, acetone, and ethanol) and then treated in oxygen plasma (90 s, 100 W, 0.4 mbar in a commercial plasma cleaner; Flecto10, Plasma Technology GmbH, Herrenberg, Germany). Afterward, they were coated by thermal evaporation with a reflection layer of 60 nm of gold (99.99%) and 3 nm of chromium (Sigma-Aldrich) as an adhesion promoter. The tipless cantilevers were coated from both sides with chromium and gold. Coating from one side only resulted in a bending of the cantilevers under small temperature changes, analogous to a bimorph, and thus led to a drift in the deflection signal during the measurements. The preparation of colloidal probes was carried out with silica particles (Bangs Laboratories, Fishers, IN) with an average diameter of 6.8 μm. These particles were attached to the cantilevers by means of a micromanipulator and UV-curable glue (Optical Adhesive 63, Norland Products, Cranbury, NJ).44,45 We used cantilevers with nominal force constants in the range of about 0.2−4 N/m. The force constants of the cantilevers were determined by the thermal noise method.46 Directly before the force measurements, the colloidal probes were cleaned analogously to the bare cantilevers with various solvents and also treated in oxygen plasma (90 s, 100 W, 0.4 mbar). Combined Setup for Electrochemistry and AFM. To determine interaction forces under potentiostatic control, a custom-made electrochemical cell was constructed for a commercial atomic force microscope (MFP-3D, Asylum Research, Santa Barbara, CA).35,36 The setup is shown schematically in Figure 1. The electrode, which was modified with a SAM, acted as the working electrode and was connected to a custom-built potentiostat based on a design from the group of H. Siegenthaler (University of Berne, Berne, Switzerland).47 In the three-electrode electrochemical cell for the AFM, a 100mm-long gold wire with a diameter of 0.25 mm acted as the counter electrode (Alfa-Aesar), and a silver wire coated with Ag/AgCl, which was placed in a circular manner around the working electrode, was used as the reference electrode. All aqueous solutions were degassed before use: first by purging with nitrogen for 15 min and then by passing through a highperformance liquid chromatography (HPLC) degassing unit placed directly before the inlet of the electrochemical cell (Gastorr BG12, Flom, Laurel, MD). The electrolyte solutions were exchanged by means of a peristaltic pump (Reglo Analog, Ismatec, Glattbrugg, Switzerland). The pH was controlled at the outlet of the fluid cell with a special pH electrode for low ionic strengths (Aquatrode Plus, Metrohm, Herisau, Switzerland) mounted in a specially constructed flow-through cell made of polypropylene that allowed working under inert gas atmosphere. Before each set of force measurements the state of the SAM on the modified gold electrodes was controlled by cyclic voltammetry (CV). The applicable potential range (i.e., the potential window in which no thiol desorption occurs) was determined in a series of experiments conducted independently from the direct force measurements presented here.38,48 During the force measurements, the current was monitored to verify that no thiol desorption was taking place. An increase in current would correspond to the formation of defects in the SAM.48−50

Additionally, for some experiments, the contact angle of the CH3-terminated SAM was controlled before (107.8° ± 0.5°) and after (105.5−107.5°) the force measurements and was found not to change significantly within the accuracy of the method. Direct Force Measurements. The force measurements were performed with an atomic force microscope equipped with closed-loop control for all three axes (MFP-3D, Asylum Research, Santa Barbara, CA). For each applied potential, a series of at least 100 approach and retraction cycles at a velocity of about 0.8 μm/s were acquired. For the force profiles of one series, no significant differences between the first and last curves could be observed. The maximum applied force was on the order of 12−15 nN. The raw data (i.e., deflection versus piezo displacement curves) were converted to force versus distance profiles by custom-written procedures in IGOR PRO (Wavemetrics) based on standard algorithms.51 In the force regime exerted by the colloidal probe, the gold electrode can be considered as incompressible, and zero separation is obtained by fitting the linear contact region of the force versus displacement curve.51 To determine the diffuse-layer potentials of the electrodes, each series of force profiles acquired for one externally applied potential was averaged before the fitting procedure.35,38 The force profiles were normalized to the effective radius44 and fitted to the full solutions of the Poisson−Boltzmann equation by means of a custom-written program in FORTRAN and IGOR PRO (Wavemetrics).52 The fits were carried out in the distance regime from one Debye length up to the onset of the regime where cantilever deflection is given by only thermal fluctuations. For an ionic strength of 1.2 mM, at which most measurements were performed, the fitting interval corresponds to distances from 12.8 to about 45−60 nm. The fitting procedure is described in more detail elsewhere.35,38,44,53,54 The potentials obtained from these fits correspond to the diffuse potentials at infinite separation of the two surfaces.52 The Debye length obtained from these fits was routinely compared to the nominal ionic strength of the electrolyte solutions. Only very seldom were deviations larger than 10% from the nominal ionic strength observed in our experiments. In this case, the data sets were excluded from further data analysis. The measurement of the interaction forces between two silica particles in the sphere/sphere geometry was performed in a commercial closed fluid cell (Asylum Research, Santa Barbara, CA) with a round glass cover slide as the bottom cover. The two particles, namely, the colloidal probe and the particle attached to the bottom glass slide, were coarsely aligned by optical microscopy; fine alignment was achieved by a procedure similar to force−volume plots.44,55 The data analysis was performed by a method analogous to that used for the direct force measurements on the electrodes.

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RESULTS AND DISCUSSION In this study, we determined the diffuse-layer properties of SAM-modified electrodes in aqueous solutions as a function of pH and externally applied potential. We concentrate on SAMs terminating in functional groups that are nonionizable and are either completely hydrophobic (CH3-terminated) or hydrophilic (OH-terminated). The diffuse-layer potentials of such electrodes can be obtained from fits to the interaction force profiles acquired by the colloidal probe technique based on AFM. However, a prerequisite for a quantitative evaluation of the interaction force profiles is accurate knowledge of the 2675

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diffuse-layer properties for the colloidal probe in the examined pH range. Based on these data, the force profiles on the electrodes can be fit afterward to the full solution of the Poisson−Boltzmann equation. The resulting potentials correspond to the diffuse-layer potentials at infinite separation of the surfaces.52 These potentials can be semiquantitatively described as functions of pH and applied potential in the framework of a recently published model that takes ion adsorption of hydronium and hydroxide ions onto the SAM into account.29,56 The adsorption of these ions influences in particular the potential of zero charge, which is the external potential at which the diffuse layer of the electrode vanishes (cf. Figure 1c). Diffuse-Layer Properties of the Colloidal Probe. Figure 2 shows some example interaction force profiles between a

κ −1 =

εε0kT 2NAe 2I

(1)

where εε0 is the total permittivity of water, kT is the thermal energy at absolute temperature, NA is Avogadro’s number, e is the elementary charge, and I is the ionic strength. The interaction forces in Figure 2 were normalized to the effective radius Reff 1 1 1 = + R eff R CP RS

(2a)

where RCP and RS are the radii of the colloidal probe and the immobilized particle as the sample, respectively. In the case of a sphere/plane geometry for the electrodes, eq 2a reduces to Reff = RCP. The force profiles F(D)/Reff can be quantitatively evaluated according to the Derjaguin equation, where Wint(D) is the free interaction energy of two infinite plates at separation D1 F(D) = 2πR eff Wint(D)

(2b)

Wint(D) results here from the overlap of the diffuse layers of the two silica particles and is given by the solutions of the Poisson− Boltzmann (PB) equation. The dashed lines in Figure 2 represent fits to the full solutions of the PB equation with the classical boundary conditions of constant charge (CC) and constant potential (CP), respectively. The solid lines represent fits to the constant-regulation (CR) approximation that takes into account the charge regulation between the two surfaces.52 It provides a better description of the interaction forces at small separation distances. In this approximation, the influence of surface chemistry is summarized by the diffuse-layer potential ψD at infinite separation and a regulation parameter p that is defined by52

Figure 2. Interaction forces between silica particles in the sphere/ sphere geometry for different pH values but constant ionic strength (I = 1.2 mM). The fits to the PB equation are represented by the dashed (CC, constant-charge conditions; CP, constant-potential conditions) and solid (CR, charge-regulation approximation) lines with respect to the different boundary conditions.

p=

colloidal silica probe and silica spheres attached to a flat substrate acquired at different pH values but a constant ionic strength of I = 1.2 mM. Such direct force measurements in the sphere/sphere geometry are necessary for determining the diffuse-layer properties of the colloidal probe in a completely symmetric system before using the probes under identical conditions to determine the unknown diffuse-layer properties of electrodes (i.e., in an asymmetric system).35,38 Each force profile shown in Figure 2 resulted from the averaging of about 50 single force versus distance curves. All measurements for different pH values were performed at a constant ionic strength of I = 1.2 mM. The absence of measurable attractive forces between silica surfaces at small separations, as would be expected from van der Waals forces, has been reported previously.44,55,57,58 It is attributed to the surface roughness of the silica particles and the presence of a gel-like layer at the silica interface. The interactions between two silica particles are, for all pH values, repulsive over the full separation distance range. The interaction force profiles can be described by the overlap of two diffuse layers originating from the surfaces of the silica particles. At separations larger than 10−15 nm, the interaction forces decay exponentially as indicated by the straight lines in the semilogarithmic representation of the force profile in Figure 2. The decay term is given by the inverse of the Debye-length κ−1 with

CD CI + CD I

(3) D

where C is the inner-layer capacitance and C is the diffuselayer capacitance. The latter is given by ⎛ eψ D ⎞ C D = εε0κ cosh⎜ ⎟ ⎝ 2kT ⎠

(4)

where ψD is the diffuse-layer potential at infinite separation of the surfaces.52 Commonly, the regulation parameter ranges from 0 to 1, where p = 0 corresponds to CP and p = 1 to CC. Charge regulation is essential for an accurate fit of the interaction force profiles between a silica colloidal probe and an electrode at small separations.35 However, the diffuse-layer potentials obtained from the PB fits are not significantly influenced by p as long as the interaction force profiles are evaluated at separation distances larger than κ−1. Variation of Colloidal Probe Properties with pH. By fitting interaction force profiles such as those shown in Figure 2, the diffuse-layer potentials ψD and the regulation parameters p can be determined for the different pH values. However, the sign of the diffuse-layer potentials cannot be inferred from the interaction forces in a symmetric system but is known to be negative.59−61 Table 1 summarizes the results for ψD and p from measurement of at least 5 different pairs of particles and two different colloidal probes for each pH. 2676

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electrodes. It should be noted that, for all of the following measurements as well, colloidal probes made from silica particles were used and the interaction forces were measured against electrodes and were thus acquired in an asymmetric system. The measurements compiled in Figure 3 were performed on two different types of electrodes that were modified with either OH-terminated SAMs (cf. Figure 3a,b) or CH3-terminated SAMs (cf. Figure 3c,d). The measurements were conducted by varying the potential applied to the electrodes for each pH value. Results for slightly acidic (i.e., pH 5.5; cf. Figure 3a,c) and slightly basic (i.e., pH 8.0; cf. Figure 3b,d) conditions are shown as examples from the full range of pH values examined (i.e., starting from pH 3.5 and extending to pH 9.5). For each pH value, a series of different potentials, ϕi , were applied to the electrode, ranging from approximately ϕ1 = −225 mV (vs SCE) to ϕ8 = +475 mV (vs SCE). For each potential ϕi , about 100 single force curves were acquired and then averaged. These potentials could vary slightly for different measurements ( κ−1) between the probe and electrode, the fits gave practically identical diffuse-layer potentials ψD, independently of the boundary conditions applied. From the fits to the PB equation at separation distances larger than κ−1 and the charge-regulation (CR) approximation, we obtained diffuse-layer potentials of ψ(ϕ1) = −16.8 mV, ψ(ϕ5) = +14.3 mV, and ψ(ϕ8) = +61.2 mV (cf. Figure 4). At the large separations D evaluated here, contributions from van der Waals interactions can be neglected. In particular, the Hamaker constant of the silica/water/electrode system was much smaller than that for a bare gold electrode because of the presence of the SAM.63 It should be noted that the van der Waals forces are attractive in our case and would thus lead to force profiles resembling those of the CP boundary condition, which was not observed experimentally in the force profiles. Diffuse-Layer Potentials versus pH. Figure 5 summarizes the diffuse-layer potentials obtained from the fits to averaged force profiles (analogous to the ones shown in Figure 4) for all pH values and applied potentials. Figure 5a shows the data for electrodes modified with an OH-terminated SAM, and Figure 5b shows the data for electrodes with a CH3-terminated SAM. Both graphs present plots of the diffuse-layer potential as a function of the applied potential. The various pH values examined in this study are represented by different symbols. The ionic strength was constant at I = 1.2 mM. Each data point in Figure 5 was obtained from at least three independent data sets acquired with different colloidal probes. 2678

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the diffuse-layer potential ψD remained practically constant and did not vary with the external potential ϕ. The variation of the data for ψD versus ϕ at different pH values (cf. Figure 5) confirms the preferential ion adsorption of hydronium or hydroxide ions on SAM-modified electrodes. The preferential adsorption of these ions was postulated theoretically27 and observed on similar SAMs, albeit without application of external potentials.6,64 At low pH, namely, high concentration of OH3+ and low concentration of OH−, the interface has a net positive charge because of the adsorption of OH3+. By contrast, at high pH, namely, high OH− and low OH3+ concentrations, the interface has a negatively charged ion layer. The adsorption of both hydronium and hydroxyl ions is more pronounced for the hydrophobic interface, which is in agreement with theoretical studies.27 Influence of the Background Electrolyte. To verify that primarily hydronium and hydroxyl ions were preferentially adsorbed to both SAMs, we also studied the influence of the background electrolyte (KCl) concentration. Figure 6 summa-

Figure 5. Diffuse-layer potential versus externally applied potential for electrodes modified with (a) OH- and (b) CH3-terminated SAMs. The data points correspond to the diffuse-layer potentials determined at different applied potentials and pH values. The lines represent the results from the global fits over all pH values to the three-capacitor model. The parameters obtained from the fits are compiled in Table 2. Figure 6. Diffuse-layer potential versus externally applied potential at pH 4.7 and different ionic strengths for an electrode with a CH3terminated SAM. The solid symbols were determined at I = 1.2 mM, whereas the open symbols were reported previously.38 The lines are based on the global fit parameters for the three-capacitor model as compiled in Table 2. The inset displays the data around the potential of zero charge (pzc).

For electrodes with an OH-terminated SAM, the diffuse-layer potentials were found to increase monotonically with applied potential for all pH values. Such monotonic increase is in agreement with the force profiles shown in Figure 3. The general dependence of ψD(ϕ) is comparable to that reported previously for OH-terminated electrodes at pH 4.7 and several ionic strengths.35,38 However, the values for the potential of zero charge (pzc, ϕpzc) shifted to higher potentials with increasing pH (i.e., increasing OH− concentration). It should be noted that this shift would be expected due to the adsorption of OH− to the SAMs as illustrated in Figure 1b. In the case of pH 4.7 (I = 1.2 mM), the value of ϕpzc = 230 ± 7 mV (vs SCE) is in good agreement with that reported previously for the same type of electrode and the same pH.35,38 For the electrodes with CH3-terminated SAMs, we found a much more pronounced variation of ψD(ϕ) as a function of pH: Only for a small range of pH values (4.7 ≤ pH ≤ 5.5) did we observe that the diffuse layer of the electrode vanished upon the application of an external potential, corresponding to the pzc. The potential regime accessible in our experiments was limited as, otherwise, desorption of the SAM occurred.48 For the CH3-terminated SAM, the pzc shifted with pH to higher values, but for more acidic (i.e., ≤pH 4.7) or basic (i.e., ≥ pH 5.5) conditions, no reversal of the sign of ψD was obtained for electrodes modified with a CH3-terminated SAM. For the acidic regime, the interaction profiles were completely attractive, whereas for the basic regime, they were completely repulsive for all external potentials ϕ. For the most basic pH (i.e., pH 9.5),

rizes the dependence of the diffuse-layer potential ψD on the applied potential ϕ for various total ionic strengths at pH 4.7 for electrodes with a CH3-terminated SAM. The overall values of ψD(ϕ) became smaller with increasing KCl concentration. This behavior is expected on basis of the Gouy−Chapman− Stern theory and was observed previously for SAM-modified electrodes.38 However, the experimentally determined potentials of zero charge (pzc) remained constant at 224 ± 15 mV and were thus practically independent of the potassium (K+) or chloride (Cl−) ion concentration. The solid lines in Figure 5 were calculated according to the three-capacitor model including ion adsorption; this model will be presented in the next section. Thus, we conclude that the effect of potassium or chloride ions can be neglected in comparison to the adsorption of hydronium or hydroxyl ions in the concentration range of KCl investigated. Otherwise, the pzc would not shift with the pH at constant KCl concentration. This observation is important, as chloride ions are known to adsorb preferentially at gold electrodes and reach saturation of Cl− on a bare gold surface already at rather low concentrations ( 8.0 for a nonpolar solid/liquid or liquid/gas interface.20 This value is comparable to that reported by Leroy et al., with log Kads,OH− = 8.94 ± 0.02 for the interface of a gas bubble in water obtained by electrokinetic measurements.21 In comparison, the values of log Kads,OH− = 4.8−6.2 for the air/water interface as reported by Manciu and Ruckenstein are somewhat lower and are thus in better agreement with our data.23,24 The log Kads,H+ adsorption constant for hydronium to hydrophobic interfaces, obtained from our fits (cf. Table 2), is consistent with other studies, where values of log Kads,H+ ≈ 1−3 have been reported.21,26 In particular, the value of log Kads,H+ ≈ 2.54 ± 0.02 as obtained by conversion of eq 14b for the dissociation constant of interfacial water is in good agreement with the value reported here in Table 2 (log Kads,H+ = 2.5) for the CH3-terminated SAM.21 However, it should be pointed out that the data from Leroy et al. originated from the electrophoretic mobility of air bubbles.21 To the best of our knowledge, no comparable data for the adsorption constants log Kads,H+ and log Kads,OH− have been

reported for hydrophilic SAMs. However, the large difference observed by us for adsorption constants on hydrophobic and hydrophilic SAMs is in good agreement with the theoretical studies of Kreutzer et al. for the same types of SAMs used here in the modification of the electrodes.27 Limitations of the Simple Ion Adsorption Model. Despite its simplicity, the model used here captures the influence of all parameters, such as the SAM termination, applied potential, and pH. However, various studies, by both theoretical and spectroscopic techniques, indicate that a more refined model for the interface between the SAM and the electrolyte must be used to provide an accurate description of the interfacial properties.23,72 Such a refined description of the interface that better takes its molecular properties into account is beyond the scope of this study. The shortcomings of our simple model are directly traceable if one replaces the global fit for ψD(ϕ) over all pH values by single fits of ψD(ϕ) for each pH value, where CSAM and Cion remain the same as stated in Table 2. These two parameters were found to have only a minor influence on the fits when varied in a physically reasonable range of values. The fits provided a far better match to the experimental data when NS, log Kads,H+, and log Kads,OH− were fitted for each pH value in an independent manner (data not shown). As expected, the number of adsorption sites was found to vary with pH for both SAMs. The corresponding values for NS ranged from 5 × 1014 cm−2 (pH 4.7) to 1 × 1014 cm−2 (pH 9.5) for the OH-terminated SAM. For the CH3-terminated SAM, they ranged from 5 × 1014 cm−2 (pH 3.5) to 0.5 × 1014 cm−2 (pH 9.5). Taking into account the different effective radii for hydronium and hydroxyl ions, such a decrease in available adsorption sites on the SAM with increasing pH is reasonable. Furthermore, the SAM structure might vary as a function of the chemical termination.73 Analogously, the adsorption constants for hydronium and hydroxyl ions were found to vary significantly when the fits were performed for each pH value separately. For the CH3terminated SAM, we found that the adsorption constant log Kads,H+ for hydronium ions covered the range of 1.5−3.5 and log Kads,OH− fell in the range of 5−8.6 for hydroxide ions. In contrast, for the OH-terminated electrode, we found that the adsorption constant for hydronium adsorption remained practically constant at a value of log Kads,H+ = 1.4 for all pH values investigated. However, log Kads,OH− varied over a rather wider range of 3.8−6.2. This variation of NS and log Kads with pH is, in our opinion, a clear indication of the shortcomings of our rather simple model. Despite its simplicity, the model used here validates ion adsorption as the primary origin for the pronounced influence of pH on the diffuse-layer properties of SAMs with nonionizable terminal groups and also captures the essential differences between hydrophilic and hydrophobic SAMs. A more accurate model must provide a better description of the adsorbed ion layer. A possible approach could include the introduction of an icelike layer at the water/solid interface, especially for the hydrophobic SAM. Such a layer has different properties than bulk water.72,74 It extends for about three to six layers of water molecules into the bulk solution and is compatible with spectroscopic studies.15,75,76 Additionally, a study by Wang and Bard based on direct force measurements with the colloidal probe technique on bare gold electrodes also raised doubts on the full validity of Gouy−Chapman theory.31 They proposed ion−ion correlations and ion condensation effects at the electrode interface not taken into account in our 2681

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with the comprehensive studies of Werner and co-workers, we also determined the diffuse-layer potentials under open-circuit conditions. Open-circuit conditions are present when the potentiostat is disconnected from the working electrode. Analogously to direct force measurements on a potentiostatically controlled electrode, the diffuse-layer potentials ψDOC can be determined by fits to the PB equation. In Table 3, the diffuse-layer potentials under open-circuit conditions (obtained by fits to the CR approximation) are compiled for an ionic strength of I = 1.2 mM and different pH values.

model. However, the additional SAM with a low dielectric constant present on the modified electrodes studied here might reduce these effects significantly with respect to those experienced by a bare metal electrode. Variation of pzc with pH. The effect of ion adsorption is most evident for the variation of the potential of zero charge (pzc) with pH (cf. Figure 1b). Figure 7 summarizes the

Table 3. Diffuse-Layer Potentials at I = 1.2 mM as Determined by Direct Force Measurements under OpenCircuit Potential Conditions ψDOC (mV) pH 3.5 4.7 5.5 8.0 9.5

Figure 7. Potential of zero charge (pzc) as a function of pH for electrodes modified with OH- and CH3-terminated SAMs (I = 1.2 mM). The lines were calculated by numerically determining the pzc for the three-capacitor model including ion adsorption with the values compiled in Table 2. The horizontal dashed lines indicate the potential range for which the SAMs were stable.

SAM−CH3 30.3 28.4 20.9 −34.2 −48.6

± ± ± ± ±

11.0 4.3 11.2 4.6 5.1

SAM−OH 22.7 19.0 −1.1 −18.9

± ± ± ±

5.8 5.3 4.7 2.9

The open-circuit potentials in Table 3 follow the same underlying mechanisms as the pzc shown in Figure 7 and the streaming potential data: Under acidic conditions, a small positive charge accumulated as a result of hydronium adsorption to the SAMs, whereas under basic conditions, a highly negative charge in the Stern layer resulted from the adsorption of hydroxyl ions. This effect was more pronounced for the hydrophobic CH3-terminated SAM. From the data in Table 3, the isoelectric point (iep), which corresponds to the pH at which the diffuse-layer charge vanishes, can be estimated. We found that the iep was between 5.5 and 8.0 for both hydrophobic and hydrophilic interfaces. Such an iep estimate is in good agreement with the findings of Dicke and Hahner based on direct force measurements of hydrophobic CH3terminated SAMs, for which an iep on the order of 6.0−7.0 was reported.41 The general trends for the open-circuit potentials as a function of pH (cf. Table 3) are also in line with the streaming potentials that were acquired without potentiostatic control. However, the streaming potential data lead to a somewhat lower iep of 3.5−4.0, which might be due to the position of plane of shear.25 The comparison of streaming potential and direct force measurements demonstrates the fundamental importance of ion adsorption for the surface charge on nonionizable SAMs.

variation of the pzc with pH for both SAMs. Solid data points indicate that the pzc was verified experimentally (cf. Figure 5), whereas open symbols indicate that the pzc should fall in a potential range where the SAMs are not stable. These data points were approximated by extrapolation of the experimental data with a fit to the three-capacitor model including ion adsorption at the corresponding pH. The potential range for the stability of the SAMs is indicated by the dashed horizontal lines with potential limits as reported in the literature.48 The solid lines were calculated by determining the zero of the threecapacitor model as given by eqs 5−15b and Table 2. However, both curves were shifted in such a way that they crossed the experimental values of the pzc at pH 5.5 for each SAM correspondingly, as those pzc values are nearest to the isoelectric point.29,56 Figure 7 illustrates that, for both types of SAMs, the pzc shifted to more positive potentials with increasing pH. This behavior of the pzc is in line with the adsorption of hydronium and hydroxyl ions onto the SAMs. However, this pzc shift was much larger for the hydrophobic methyl-terminated SAMs than for the hydrophilic hydroxyl-terminated SAMs. Ion adsorption leads to an increasing charge in the Stern layer, which has to be compensated at the pzc by larger externally applied potentials of opposite sign to counterbalance the charge in the Stern layer. It should be noted that the determination of the pzc is also possible by “classical” electrochemical techniques, such as the determination of the diffuse-layer capacitance by cyclic voltammetry or transient techniques.39 Thus, measuring the pzc on modified electrodes provides an alternative experimental approach in following ion adsorption processes on modified electrodes. Streaming potential measurements represent an alternative method for studying ion adsorption processes on solid substrates, particularly for SAMs.6,25,26 To compare our results

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CONCLUSIONS Direct force measurements on electrodes modified with SAMs provide a direct approach to characterize ion adsorption phenomena at nonionizable, molecularly defined interfaces. Unlike for electrokinetic methods, the diffuse-layer properties of such SAM-modified electrodes can be quantitatively determined without taking into account additional parameters such as the position of the shear plane or surface conductivity. A simple model based on hydronium and hydroxyl ion adsorption at the SAMs allows for the description of the main dependencies in terms of the chemical functionality of the SAM, pH, and external potential. In particular, the potential of zero charge, as the externally applied potential at which the charge of adsorbed ions is counterbalanced, allows for the 2682

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unambiguous identification of ion adsorption. We were able to demonstrate that ion adsorption at SAMs is much more pronounced on hydrophobic substrates than on hydrophilic ones, in agreement with theoretical studies.27 Ion adsorption is dominated by hydroxyl ions and only to a lower extent by hydronium ions, whereas the influence of the background electrolyte KCl can be neglected. However, this might be not the case for other background electrolytes, especially those containing multivalent ions. Our findings are of direct relevance to the use of SAMs in sensors, such as plasmonic sensors, where hydrophilic or hydrophobic SAMs are often used as passivation layers to cover the areas left free by the specific binding sites, for example, cysteine-attached DNA segments.77,78 Furthermore, the approach presented herein allows spectroscopic and electrokinetic techniques to be complemented with an additional parameter, namely, the externally applied potentials, to determine ion adsorption with higher sensitivity. Because of the potentiostatic control, ion adsorption can be probed over a wide range of surface properties. At the same time, parameters such as the pzc can also be determined by classical electrochemical techniques, such as diffuse-layer capacitance measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 921 55 2335. Present Address †

Analytische Chemie − Elektroanalytik & Sensorik, RuhrUniversität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Michal Borkovec, Hans Siegenthaler, and Samuel Rentsch for fruitful discussions and, in particular, Samuel Rentsch for the initial experiments that led to this study. This research was supported by the Swiss National Science Foundation (SNSF) and the German Research Foundation (DFG SFB 840).

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